1. Field of the Invention
The present invention relates to an optical information recording medium capable of recording/reproducing information signals by irradiating high energy light beam such as laser onto a thin film formed on a substrate, its manufacturing method, a manufacturing apparatus, a recording/reproducing method, and a recording/reproducing apparatus.
2. Description of the Related Art
Research and development are intensive about optical information recording media for irradiating laser beam onto the medium formed by laminating thin films on a plane of a substrate preliminarily forming grooves for guiding laser beam to heat locally, changing in optically detectable two or more states formed by the different light irradiating conditions, and recording information by corresponding this change to the information signal. Basic means for increasing the quantity of information to be accumulated per piece of optical information recording medium includes a method of enhancing the recording plane density by reducing the spot diameter of laser beam by shortening the wavelength of laser beam or increasing the numerical aperture of the objective lens for focusing it.
The recent main stream is the use of optical system of wavelength of 660 nm and numerical aperture of about 0.6 as represented by a recordable DVD. It has been further studied to enhance the numerical aperture to about 0.85 by using a blue laser diode near wavelength of 400 nm which is presently in a practical stage. By heightening the numerical aperture, the allowable width to tilting of optical disc becomes smaller; hence, it has been also proposed to reduce the thickness of transparent substrate at the laser beam incident side from 0.6 mm of recordable DVD to about 0.1 mm.
Further, in order to increase the quantity of information accumulated per medium, there has been proposed to laminate a plurality of layers for recording/reproducing information as multilayer structural medium (for example, see JP-A 2000-36130, pp. 7-8, FIG. 1). In a multilayer recording medium, an information layer near the laser light source absorbs light, and an information layer remote from the laser light source records/reproduces information by an attenuated laser beam. Accordingly, in the multilayer recording medium, the information layer near the laser light source is higher in transmissivity, and the information layer remote from the laser light source is higher in reflectivity, reflectivity difference and sensitivity, so that a sufficient recording/reproducing characteristic may be obtained by a limited laser power.